Method for sintering workpieces of pressed powdered refractory metal or alloy and vacuum furnace for performing the same

ABSTRACT

A method for sintering workpieces of a pressed powdered refractory metal in vacuum. The method envisages simultaneous heating of the workpiece by electron bombardment over its entire surface. The workpiece to be treated is heated to a temperature which makes from 10 to 15 per cent of its material melting temperature and is at such temperature till gases cease to evolve therefrom. A vacuum furnace for performing the method has a heater made as a cathode surrounding the entire surface of the workpiece to be treated and placed at a uniform distance from the workpiece.

United States Patent [1 1 Borok et a1.

[ METHOD FOR SINTERING WORKPIECES OF PRESSED POWDERED REFRACTORY METAL OR ALLOY AND VACUUM FURNACE FOR PERFORMING THE SAME [76] Inventors: Boris Alexandrovich Borok; Zhan losifovich Dzneladze; Leonid Nikolaevich Petrov; Evgeny Ivanovich Zaikin; Ivan Alexeevich Klynnsky; Boris Pavlovich Lobashov, all of Moscow, USSR.

[22] Filed: May 19, 1971 [21] Appl. No.: 144,719

[52] US. Cl 75/225, 75/200, 219/121 EB, 219/121 EM, 250/49.5 TE, 266/5 R, 266/24 [51] Int. Cl. B22f l/00, B23k 9/00, H011 37/26 [58] Field of Search 75/225, 200; 219/121 EBM, 121 EBA; 250/495 TE,

49.5 R; 266/24, 2 R 5 R [56] References Cited UNITED STATES PATENTS 2,227,177 12/1940 Berghaus et a1. 75/225 X 3,381,157 4/1968 Ferreira 313/348 3,183,086 5/1965 Kurtz et a1. 75/208 R X 2,994,801 8/1961 Hanks 219/121 EBA 1 Oct. 30, 1973 6/1960 Smith ..219/121EBM 2,942,098 2,809,905 10/1957 Davis et al 219/121 EBM FOREIGN PATENTS OR APPLICATIONS 37/1455 8/1962 Japan 75/225 OTHER PUBLICATIONS Brown, M. J., Western Electric Technical Digest, No. 15, July 1969, p. 25-26.

Primary Examiner-Carl D. Quarforth Assistant ExaminerR. E. Schafer Attorney-WatemRoditi, Schwartz & Nissen [57] ABSTRACT A method for sintering workpieces of a pressed powdered refractory metal in vacuum. The method envisages simultaneous heating of the workpiece by electron bombardment over its entire surface. The workpiece to be treated is heated to a temperature which makes from 10 to 15 per cent of its material melting temperature and is at such temperature till gases cease to evolve therefrom.

A vacuum furnace for performing the method has a heater made as a cathode surrounding the entire surface of the workpiece to be treated and placed at a uniform distance from the workpiece.

3 Claims, 2 Drawing Figures PAIENIEU um 30 I915 sum 1 BF 2 FIG. 2

METHOD FOR SINTERING WORKPIECES OF PRESSED POWDERED REFRACTORY METAL OR ALLOY AND VACUUM FURNACE FOR PERFORMING THE SAME The present invention relates to methods for sintering workpieces of pressed powdered refractory metal and to a vacuum furnace for effecting the process.

A method for sintering workpieces of pressed powdered refractory metal or alloy is known, which comprises heating the workpiece by electron bombardment of its surface in a vacuum, the workpiece being moved in the focused electron beam.

The sintering process is a vacuum furnace with a heater emitting an electron beam is a very slow one because the workpiece is heated gradually over small areas. The gases released from the heated area are partially absorbed by the previously heated areas. Additionally workpieces different in thickness and form cannot be heated evenly. Therefore only small flat and thin workpieces are treated using means for moving them in the focused beam of an electron gun. The workpiece areas to be treated are heated to a sintering temperature without retention it at intermediate temperatures; some gas has no time to escape from the workpiece before its shrinkage, which results in the formation of deep cracks on its surface and blowholes inside.

The object of the present invention is to overcome the disadvantages of the above described method and apparatus known in the art.

A specific object of the invention is to provide a method for sintering workpieces of pressed powdered refractory metal speeding up the sintering of the workpieces and improving their quality as well as ensuring the sintering of large articles of intricate shape.

These objects are achieved by providing a method for sintering workpieces of pressed powdered refractory metal, comprising heating the workpiece by electron bombardment of its surface in a vacuum, whereby, according to the invention, the entire surface of the workpiece is subjected to the electron bombardment simultaneously.

The method according to the invention allows speeding up the sintering process and sintering large intricate workpieces.

it is preferable to keep a workpiece heated to a temperature which makes from to 1-5 percent of its material melting temperature till the gas ceases to evolve from it.

This ensures a high quality of the articles, that is, by avoiding the formation of blowholes and deep cracks on their surfaces.

For performing this process, a vacuum furnace has been developed with a heater emitting an electron flux; according to the invention the heater is made as a cathode surrounding the entire surface of the workpiece being treated.

This furnace ensures a higher output and a higher quality of the articles obtained.

Preferably, the heater surrounding the workpiece to be treated is uniformly spaced from its surface.

This allows improvement of the quality of the intricate workpieces to be treated.

The invention will be more clearly understood by way of example with reference to the accompanying drawings, in which:

FIG. 2 is a sectional view of the furnace taken along.

the lines llll of FIG 1.

The vacuum furnace for performing themethod for sintering workpieces according to the present invention comprises a cylindrical water-cooled housing 1 with a cover 2 and a lifter 3. Inside the housing 1 on highvoltage insulators 4 there are mounted shields 5 made from sheet molybdenum. A heater 6' attached to electric lead-ins 7 is made as a cathode of tungsten or molybdenum sheets or wire which surrounds the entire surface of the workpiece 9. The heater 6 is coupled to heating and rectified high voltage-sources (not shown). A table 8 supports'the workpiece in a predetermined position and is driven vertically through a vacuumseal 10. The furnace housing 1 and the supporting table- 8 are grounded/For providing vacuum conditions in therange of 10" mm of mercury, the furnace is equipped with a high vacuum oil-free pump backed with a mechanical pump (not shown in the drawing). An optical pyrometer 11 and thermocouples (not shown) are provided for measuring the workpiece temperature, as well as the temperature of separate elements of the furnace.

In the operation of the apparatus described hereinbefore the cover 2 is opened by the lifter 3. The pressed workpiece 9 is lowered onto the table 8. The heater 6' is placed around the workpiece at an equal distance from its surface. Then the cover 2 is lowered and the furnace is hermetically closed. The vacuum pumping equipment (not shown) is put into operation to provide the vacuum conditions required. Further the source of the heating voltage is switched on (not shown) and the cathodeheater 6 is heated to a temperature at which electron emmission commences from its surface. Then the source of rectified high voltage is switched on (not shown) and a negative potential is applied to the cathode. Thus a fluxof electrons from the cathode is directed to the entire surface of the workpiece which becomes an anode. The surface of the workpiece is heated by the impingement of electrons upon it. By the electron bombardment, the workpiece is heated up to the melting temperature, which makes from 10 to l5 per cent of its material melting temperature, and is kept at this temperature till the gases cease to evolve therefrom. The holding time depends on the amount of volatile components in the powder. Further heating of the workpiece right up to the sintering temperature is carried out at a rate of 50/min. After the isothermic holding at the sintering temperature, the heating of the cathode is discontinued and the high-voltage source. is disconnected. After the cooling of the workpiece down to room temperature, the furnace housing is unsealed and the workpiece removed from the heater.

When high-voltage insulator 4 is used instead of seal 10, a high positive potential may be applied to the table 8, and the housing 1, shields 5 and the heater 6 are then grounded. The principle of the sintering method remains the same because in this case the cathode heater 6 will have a negative potential with respect to the anode the workpiece 9. The application of this mode of furnace electric supply allows separation of the sources of heating and rectified high voltages which simplifies the operation of the furnace.

The method has been tested for sintering workpieces of powdered tungsten, molybdenum, columbium and of other refractory metals and alloys.

The method according to the present invention allows obtaining practically porous-free sintered articles from the above-mentioned metals up to 250 mm in diameter and up to 400 mm in height. The articles contained insignificant amount of gaseous components uniformly distributed therein.

What we claim is l. A method for sintering workpieces of a pressed powdered refractory metal comprising providing a vacuum of about mm of mercury around said workpiece, heating the workpiece by simultaneous electron bombardment of its entire surface, from a heated cathode, up to a temperature which makes from 10 to per cent of its material melting temperature, holding the workpiece at such temperature until gases cease to evolve from the workpiece, and then heating the workpiece to a complete sintering temperature.

2. A vacuum furnace for sintering workpieces, comprising a table for supporting the workpiece to be treated; means movably supporting the table inside said furnace; a heater constituted as a cathode surrounding the entire surface of said workpiece, said heater comprising a plurality of parallel elements encircling the workpiece in facing relation at a substantially uniform spacing therefrom; and an electrical lead-in connected with said heater and with sources of high and heating voltages.

3. A vacuum furnace as claimed in claim 2 wherein said parallel heater elements extend substantially beyond the ends of the workpiece for heating the end faces of the workpiece. 

2. A vacuum furnace for sintering workpieces, comprising a table for supporting the workpiece to be treated; means movably supporting the table inside said furnace; a heater constituted as a cathode surrounding the entire surface of said workpiece, said heater comprising a plurality of parallel elements encircling the workpiece in facing relation at a substantially uniform spacing therefrom; and an electrical lead-in connected with said heater and with sources of high and heating voltages.
 3. A vacuum furnace as claimed in claim 2 wherein said parallel heater elements extend substantially beyond the ends of the workpiece for heating the end faces of the workpiece. 